The present invention relates to an oscillator circuit formed in a semiconductor integrated circuit (IC), specifically to a stabilized oscillator circuit in which the oscillation frequency variations due to power source fluctuations are suppressed. The oscillator is used, for example, in an FM radio receiver circuit incorporated in a battery-driven portable device.
In the conventional art, the local oscillator circuit of the FM radio receiver of a battery-driven portable device, such as a portable tape recorder or a portable CD player, is formed as part of a semiconductor integrated circuit. A specific example of such a local oscillator circuit is a differential amplifier type oscillator circuit shown in FIG. 9, or a Colpitts oscillator. As can be seen from these examples, the local oscillator is comparatively simple in configuration.
In the case of the differential amplifier type oscillator circuit shown in FIG. 9, an oscillation signal from a differential amplifier type oscillator circuit 91 is output after being amplified by a differential amplifier type buffer output circuit 92.
The differential amplifier type oscillator circuit 91 is made up of: NPN transistors Q1 and Q2 which constitute a differential amplifier circuit, with their bases and collectors cross-connected (that is, the base and collector of one transistor are connected to the collector and base of the other transistor, respectively); a first constant current source I1 connected between the common emitter node of the NPN transistors Q1 and Q2 and a ground potential node; and a resonator circuit 90 connected between a power supply node and the collector of NPN transistor Q2 (the inductor component of the resonator circuit is denoted by L, and the capacitor component is denoted by C). The collector of NPN transistor Q1 is connected to the power supply node.
The resonator circuit 90 is normally a resonator provided as an external component of an IC, and the circuits other than the resonator circuit 90 are incorporated inside the IC.
The differential amplifier type oscillator circuit 92 is made up of: NPN transistors Q3 and Q4 which constitute a differential amplifier circuit; a second constant current source I2 connected between the common emitter node of the NPN transistors Q3 and Q4 and a ground potential node; and resistors R1 and R2 connected at one end to the collectors of the NPN transistors Q3 and Q4, respectively and at the other end to the predetermined single node.
The bases of NPN transistors Q3 and Q4 are connected to the bases of NPN transistors Q2 and Q1, respectively.
In FIG. 9, reference symbol Cs denotes a parasitic capacitance existing between the collector of the NPN transistor Q2 of the oscillator circuit 91 and a ground potential node. The parasitic capacitance is an equivalent representation of a synthetic capacitance, including mainly the collector-ground capacitance of NPN transistor Q2 and the base-collector capacitance Ccb4 (mirror capacitance) of NPN transistor Q4. The parasitic capacitance is AC-connected in parallel to the capacitive component C of the resonator circuit 90.
In the differential amplifier type oscillator circuit described above, the parasitic capacitance Cs is connected in parallel to the capacitance of the resonator circuit 90.
With this circuit configuration, if the collector potentials of the transistors Q2 and Q4 lower due to fluctuations in the power supply voltage, the parasitic resistance Cs increases, and the resonance characteristics of the resonator circuit 90 vary in such a manner that the resonance frequency f becomes lower. Conversely, if the collector potentials of the transistors in use rise, the parasitic resistance Cs decreases, and the resonance characteristics of the resonator circuit 90 vary in such a manner that the resonance frequency f becomes higher.
In summary, the exhaustion of a battery or the rotation of the motor of an oscillator circuit-incorporated device give rise to ripples superimposed on a power supply voltage, and the resultant fluctuations in the power supply voltage result in variations in the oscillation frequency.
As described above, the conventional oscillator circuit entails the problem that the level of the oscillation frequency varies, dependent on the fluctuations in the power supply voltage.